The present invention is a method and apparatus for developing sheets of thermophotographic or heat developable film.
Thermophotographic film typically includes a thin polymer or paper base coated with an emulsion of dry silver or other heat sensitive material. Once the film has been imaged, it is developed through the application of heat. Devices and methods for developing thermophotographic film are generally known and disclosed, for example, in the following U.S. Patents:
______________________________________ Inventor U.S. Pat. No. ______________________________________ Svendsen 3,629,549 Brewitz 3,648,019 Kreitz et al. 3,709,472 Svendsen 4,518,845 ______________________________________
The Svendsen U.S. Pat. Nos. 3,629,549 and 4,518,845 both disclose developers having thermally insulating drums concentrically mounted within a heating member. Sheets of film to be developed are engaged by the drum and driven around the heating member. Unfortunately, developers of this type are relatively complicated and poorly suited for use with film having soft emulsions. Since the side of the film bearing the emulsion will contact either the insulating drum or the heating member, the film is subject to damage by sticking or scratching.
The development device disclosed in the Kreitz et al. U.S. Pat. No. 3,709,472 uses a heated drum to develop strips of film, and is not suitable for single sheets of film having soft emulsion layers.
The Brewitz U.S. Pat. No. 3,648,019 discloses a developer with a pair of heaters on opposite sides of a low thermal mass locating device such as a screen assembly. Although it is portable, this developer is relatively slow and poorly suited for commercial applications.
Other thermophotographic film developers include a heated drum which is electrostatically charged to hold the film thereon during development. Since the, side of the film bearing the emulsion is not in contact with the drum or other developer components, it is not subject to sticking or scratching as in some of the developers discussed above. Unfortunately, the electrostatic system used to hold the film on the drum during development is relatively complicated and poorly suited for developers configured to develop larger sized sheets of film.
The 3M Model 261 and 262 thermal diazo processor system uses a belt to transport the film as it is being heated. The belt is a relatively hard, polytetrafluoroethylene (PTFE) coated fiberglass member.
The 3M Model 1500 thermal diazo processor develops rolls of film by transporting the film over a hot drum, in a manner similar to that disclosed in the Kreitz et al. patent discussed above.
In general, and as is discussed in the background sections of the patents referenced above, the density of the developed image is dependant upon the amount of heat to which the film emulsion is exposed. Nonuniform heating ("hot spots") can produce an uneven developed image density. Uneven physical contact between the film, and any supporting structures during the development process can also produce visible marks and patterns on the image.
It is evident that there is a continuing need for improved thermophotographic film developers. In particular, there is a need for a developer capable of quickly and uniformly developing large sheets of film without damaging the emulsion. To be commercially viable, any such developer must be capable of being efficiently manufactured.